One form of process for the production of hollow bodies or articles from oriented thermoplastic material comprises taking a hollow preform which has a bottom, heating the cylindrical portion of the preform and then subjecting it to a stretching operation at a starting temperature which is below the vitreous transition temperature, with the wall thickness of the preform being reduced at the same time. Such a process is used, preferably when employing preforms of linear polyesters, for example polyethylene teraphthalate (PETP) or polylutylene teraphthalate (PBTP), for example when there is a need to produce containers for materials which are under a high pressure, such as for example drinks containing carbon dioxide gas. The operating procedure involved in such a process is generally such that the heated preform which is of a generally cup-like configuration and which may be produced for example by injection molding to constitute a parison member is stretched over a mandrel whose length corresponds at least to that of the preform when stretched and whose outside diameter substantially corresponds to the inside diameter of the preform which is substantially the same as that of the article when in the stretched condition. The stretching operation is generally carried out in such a way that, starting from the bottom end of the preform, a drawing ring is moved along the cylindrical wall portion of the preform, towards the free end thereof. During that movement of the drawing ring, the wall portion along which the ring moves is stretched for example to three to four times its initial length, while at the same time the wall thickness thereof is also substantially reduced. The mandrel employed within the preform performs inter alia the function of supporting the wall poriton of the preform which is to be stretched in that way in order thereby to prevent a reduction in the diameter thereof during the stretching operation.
In order to achieve a high level of output in that procedure, it is desirable for the stretching operation to be carried out at the highest speed that can be reasonably attained in order thereby to minimise the amount of time required for thatpurpose. On the other hand however it is not possible to increase the rate at which the stretching effect is produced, to just any random value, as that involves the risk of adversely affecting the quality of the end product. An aspect which is of importance in that connection is that the temperature of the preform and the admissible speed of stretching in the preform stretching operation are related insofar as the stretching speed can be increased with increasing temperature. Although it is possible therefore to stretch the preform at ambient temperature, that can only be done at a very low speed. The use of higher speeds of stretching requires the preform to be heated beforehand, but the temperature thereof should be below the vitreous transition temperature. Another point to be taken into consideration in that connection is that during the stretching operation, due to the work involved in changing the shape of the preform, the preform experiences an additional rise in temperature, the extent of which is also dependent on the speed of the stretching operation. Accordingly, when heating the preform prior to carrying out the stretching operation, the procedure is to be such that the temperature which obtains after the heating operation is below the vitreous transition temperature to such an extent that, even after the following rise in temperature due to the work involved in transforming the preform into the finished article, during the stretching operation, at least the predominant part of the wall of the preform which is to undergo stretching is still below the vitreous transition temperature.
The initial phase of the stretching operation requires particular care to be taken. That applies in particular in regard to the commencement of the initial phase, that is to say, when the drawing ring begins to produce its effect on the preform. In that part of the operation it is necessary in any case to effect stretching at a lower rate so that the flow of the material forming the preform, which is necessary to give the desired effect, that is to say, to produce the necessary orientation of the molecules in the preform material, is caused to begin in such a manner as to avoid overloading of the material, which could result in defects in the finished product or which could also result in the preform becoming torn apart during the stretching operation. It is further necessary to ensure that the initial phase of the stretching operation and in particular the commencement thereof takes place in the same manner from one working operation to the next, that is to say from one preform to another, in order thereby to give product which have the same properties. If the force for carrying out the drawing operation by moving the drawing ring along the preform were to be applied abruptly to the preform, that could give rise to irregularities and lack of uniformity in the finished article, in regard to the portion of the preform at which the flow of the preform material begins and also in respect of the way in which material flow is caused, in particular in the initial phase. A further point to observe in that connection is that the region of the preform at which the stretching operation begins does in any case represent the transitional region between the cylindrical wall portion which is to be stretched by the action of the drawing ring, and the bottom of the preform, which does not undergo any stretching effect. It will be appreciated that the above-mentioned transitional region in which different parts of the preform, namely the bottom and the cylindrical wall portion thereof, which have different properties and are possibly also of different thicknesses, blend into each other, is particularly critical in regard to the quality of the end product so that for that reason also it is necessary to provide for a defined and reproducible initial phase in the stretching operation in order to arrive at a given and defined result.